Aircraft are generally required to have certain directional stability and control characteristics. In order to achieve these characteristics, aircraft generally have vertical stabilizers and rudders. Often, the size of the vertical stabilizers, the size of the rudder(s), and the power of the rudder actuators (including the size and/or operating pressure of the associated hydraulic system(s)) of large multi-engine aircraft is determined by certain engine-out controllability requirements. For example, during aircraft certification, government agencies (e.g., the Federal Aviation Administration) often require manufacturers to determine minimum control speeds on the ground and in flight for engine failures at selected conditions.
The minimum control speed in-flight can include a calibrated airspeed at which, when a critical engine is suddenly made inoperative, it is possible to maintain control of the airplane during specific operating conditions. The minimum control speed on the ground can include a calibrated airspeed during the takeoff run at which, when a critical engine is suddenly made inoperative during selected operating conditions, it is possible to maintain control of the airplane using the rudder control alone while using normal piloting skills. In general, the higher the minimum control speeds an aircraft has, the more runway the aircraft requires for takeoff and/or landing. Accordingly, it is desirable to design aircraft to have low minimum control speeds.
Low minimum control speeds generally require large vertical stabilizing surfaces, larger rudder surfaces, and powerful rudder actuators (e.g., fast moving and/or high force rudder actuators). Larger surfaces and/or powerful actuators increase the weight of an aircraft and in many cases increase the complexity and manufacturing costs. Additionally, larger surfaces can increase drag during various phases of flight (e.g., during cruise flight) resulting in increased fuel usage and costs.
As described in U.S. Pat. No. 5,375,793, issued Dec. 27, 1994, which is fully incorporated herein by reference, manufacturers have considered using a fly-by-wire flight control system to momentarily deflect an aileron and/or a spoiler to create drag on a wing to cause a yawing moment to supplement the yawing moment created by the rudder during certain engine-out conditions. Accordingly, the yawing moment is proportional to the amount of drag created and the distance between where the drag force is applied and the aircraft's center of gravity (“c.g.”). In order to increase the yawing moment the drag must be increased and/or the force must be applied further from the c.g. A problem with this solution is that increases in drag reduce the thrust-to-drag ratio of the aircraft, which can degrade engine-out performance (e.g., the ability for the aircraft to accelerate). Another problem with this solution is that creating the drag force further from the aircraft's c.g. requires a spoiler or aileron deflection on the outboard portion of the wing, which in many cases can create a rolling moment on the aircraft. In order to compensate for this rolling moment, other flight control surfaces must be deflected, which can increase pilot workload, increase flight control complexity, and/or add to overall aircraft drag.